The invention relates to an improved method for the simultaneous removal of SO.sub.2 and NO from flue gases.
Typically, flue gases resulting from the combustion of carbonaceous material in power plants contain hundreds to thousands of parts per million (ppm) of SO.sub.2 and several hundred ppm of NO.sub.x, mostly in the form of NO.
Removal of the sulfur dioxide from flue gas can be accomplished by wet scrubbing. By this method, the flue gas is passed through an aqueous slurry of lime or limestone which reacts with the SO.sub.2 to form insoluble CaSO.sub.4 and CaSO.sub.3 species. In a variation, known as the dual alkali method, the flue gas is passed through a scrubber solution containing 0.05 to 1.5 gm moles per liter of an aqueous sulfur dioxide absorbant such as sodium carbonate which reacts with the S.sub.2 in solution to form NaHSO.sub.3. The scrubber solution is regenerated by contact with a lime or limestone slurry where the NaHSO.sub.3 reacts with the calcium forming the insoluble hydrated CaSO.sub.3 and CaSO.sub.4 species which are separated and sent to landfill while the regenerated sorbent solution is recycled back to the scrubber.
For some time attempts have been made to find a process, compatable with wet flue gas scrubbing, which will also remove NO from the flue gas. Teramoto et al reported in the Journal of Chemical Engineering of Japan, Vol. 11, No. 6 (1978) on the use of ferrous ethylenediaminetetaacetic acid, hereinafter Fe(II)EDTA, to absorb NO into an aqueous solution. While it was hypothesized that sulfur dioxide participated in the removal of NO, the mechanism proposed did not identify the NO and SO.sub.2 reaction products and failed to appreciate the significance of the bisulfite ion in promoting the NO removal. Furthermore, these investigators did not operate with oxygen and other major flue gas components in their experimental system which have a major impact on the actual removal of NO from flue gas.
Additional work in the United States on the use of metal chelates such as Fe(II)EDTA on wet flue gas scrubbing has been reported by Chang in Environmental Science Technology Vol. 17, No. 11, 1983. This work also indicated that the metal chelates might be effective in the simultaneous removal of SO.sub.2 and NO from gas streams, however once again the work was done under oxygen-free conditions.
Flue gases generally contain O.sub.2 and, prior to this work, investigations into the use of scrubber solutions containing Fe(II)EDTA for removing NO from flue gas streams have shown that the presence of oxygen in the flue gas quickly decreased the removal efficiency of the scrubber solution from about 80 to about 20% of the NO present in the stream.
We have developed an improved method for the simultaneous removal of SO.sub.2 and NO from flue gases which is compatable with present wet flue gas desulfurization methods. Furthermore, we have developed a denitrification process, that operates in conjunction with the desulfurization processes by utilizing metal chelates for removing NO from flue gases which also contain oxygen.